As is well known, there is available, as one type of steam turbine, that which is provided with a casing, a shaft body (rotor) installed inside the casing so as to be rotatable, a plurality of stationary blades arranged by being fixed to an inner circumference of the casing and a plurality of moving blades installed in a radial pattern on the shaft body on the downstream side of the plurality of stationary blades (downstream side of the flow of steam). Of these steam turbines, an impulse turbine converts pressure energy of steam to velocity energy by stationary blades and also coverts the velocity energy to rotational energy (mechanical energy) by moving blades. Further, a reaction turbine is that in which pressure energy is converted to velocity energy also inside moving blades and the velocity energy is converted to rotational energy (mechanical energy) by reaction force derived from ejection of steam.
In the above-described types of steam turbines, normally, a space is formed in the radial direction between the leading end of a moving blade and a casing which surrounds the periphery of the moving blade to form a flow channel of steam. A space is also formed in the radial direction between the leading end of a stationary blade and a shaft body. However, leakage steam passing to the downstream side through the space at the leading end of the moving blade will not impart torque to the moving blade. Further, in leakage steam which passes to the downstream side through the space at the leading end of the stationary blade, pressure energy is not converted to velocity energy by the stationary blade. Therefore, torque is hardly imparted to the moving blade on the downstream side. Therefore, in order to improve the performance of a steam turbine, it is important to reduce the amount of leakage steam which passes through the space at the leading end of the moving blade.
Patent Document 1 given below has proposed a structure which is provided at the leading end of a moving blade with stepped parts which are gradually increased in height from the upstream side toward the downstream side of the flow of steam (fluid) flowing in the axial direction (a direction parallel to a rotating shaft of the moving blade). In this structure, seal fins which have spaces with respect to the stepped part are provided.
According to such a configuration, the leakage flow of steam which has passed through the spaces of the seal fins collides with an end edge which forms a step surface of the stepped part to result in an increase in flow resistance of steam. Thereby, steam is reduced in the leakage flow rate.